Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper.
Early inkjet printers used a single monochromatic pen, typically carrying black ink. Later generations of inkjet printing mechanisms used a black pen which was interchangeable with a tri-color pen, typically one carrying the colors of cyan, magenta and yellow within a single cartridge. Here, the service station was designed to service either type of cartridge.
The next generation of printers further enhanced the images by using either a dual pen system or a quad pen system. The dual pen printers provided a black pen along with a tri-color pen, both of which were mounted in a single carriage. Here, the service stations had caps arranged side-by-side to simultaneously seal both the black and tri-color printheads. These dual pen devices had the ability to print crisp, clear black text while providing full color images. The quad pen printing mechanisms had a first pen carrying black ink, a second pen carrying cyan ink, a third pen carrying magenta ink, and a fourth pen carrying yellow ink. Quad pen plotters typically carried four cartridges in four separate carriages, so each cartridge needed individual servicing. Quad pen desktop printers were designed to carry four cartridges in a single carriage, so all four cartridges could be serviced by a single service station.
These earlier dual and quad pen printers required an elaborate capping mechanism to hermetically seal each of the printheads during periods of inactivity. A variety of different mechanisms have been used to move the servicing implements into engagement with respective printheads. For example, a dual printhead servicing mechanism which moves the caps in a perpendicular direction toward the orifice plates of the printheads is shown in U.S. Pat. No. 5,155,497, assigned to the present assignee, Hewlett-Packard Company, of Palo Alto, Calif. Another dual printhead servicing mechanism uses the carriage to pull the caps laterally up a ramp and into contact with the printheads, as shown in U.S. Pat. No. 5,440,331, also assigned to the Hewlett-Packard Company. A rotary device for capping dual inkjet printheads is commercially available in several models of printers produced by the Hewlett-Packard Company of Palo Alto, Calif., including the DeskJet.RTM. 850C, 855C, 820C and 870C model printers. Examples of a quad pen capping system that use a translation motion are seen in several other commercially available printers produced by the Hewlett-Packard Company, including the DeskJet.RTM. 1200 and 1600 models. Thus, a variety of different mechanisms and angles of approach may be used to physically move the caps into engagement with the printheads.
The caps in these earlier service station mechanisms typically included an elastomeric sealing lip supported by a movable platform or sled. This sled was typically produced using high temperature thermoplastic materials or thermoset plastic materials which allowed the elastomeric lips to be onsert molded onto the sled. The elastomeric sealing lips were sometimes joined at their base to form a cup-like structure, whereas other cap lip designs projected upwardly from the sled, with the sled itself forming the bottom of the sealing cavity. Typically, provisions were made for venting the sealing cavity as the cap lips are brought into contact with the printhead. Without a venting feature, air could be forced into the printhead nozzles during capping, which could deprime the nozzles.
Capping systems need to provide an adequate seal while accommodating a several different types of variations in the printhead. For example, today's orifice plates often each have a waviness. Commercially available orifice plates are not perfectly planar, but they may be slightly bowed in a convex, concave or compound (both convex and concave) configuration. This waviness may generate a height variation of up to 0.05-0.08 millimeters (2-3 mils; 0.002-0.003 inches). These orifice plates may also have some inherent surface roughness over which the cap must seal. The typical way of coping with both the waviness problem and the surface roughness problem is through elastomer compliance, where a soft material is used for the cap lips. The soft cap lips compress and conform to seal over these irregularities in the orifice plate.
Another feature shared by the earlier capping systems is the ability to accommodate planar misalignments between the orifice plates of cartridges installed in a printing mechanism. Due to various manufacturing tolerances associated with the pen carriage and the pens themselves, as well as minor variations in the placement of the cartridges within the carriage, the sealing surfaces of adjacent orifice plates may not lie the same plane. Indeed, the planes defined by these orifice plates may lie at a variety of different angles with respect to one another. Moreover, the sealing surface of an individual pen may not lie in a single plane. Thus, a capping system must be able to accommodate these different types of irregularities. Minor irregularities are accommodated by the elastomeric nature of the sealing lips, which allows the lips of a single cap to be compressed more in one area than in another.
These planar misalignments, where the orifice plates are at different heights and/or tilted with respect to a reference plane, were traditionally addressed by using elaborate mechanisms. Typically these mechanisms had spring-loaded cap sleds to accommodate for the height variation, with the sleds also having a gimbaling feature so they could tilt to seal a tilted orifice plate. Some of the later service stations, such as the rotary capping device commercially available in the DeskJet.RTM. 850C, 855C, 820C and 870C model printers produced by the Hewlett-Packard Company, use a coiled spring underneath the capping sled, with the spring being compressed when the printheads are capped. Other mechanisms have mounted the printhead caps on separate arms, for example, as commercially available in the DeskJet 660C model color inkjet printer sold by the Hewlett-Packard Company. Each arm has one end pivotally attached to the frame, with a cap base pivotally attached to the other end of the arm. Each arm is biased toward the printhead by a spring which is compressed during capping. Unfortunately, such earlier spring mechanisms for accommodating printhead-to-printhead planar misalignments were often elaborate and required many different parts to be assembled into the final capping unit. These additional parts increased the overall cost of the inkjet printer, not only in material costs, but also in labor costs required for assembly.
Another shortfall of the earlier multi-pen capping systems was the physical width required to place each cap side-by-side on the capping sled. For example, when onsert molding the cap lips to a plastic sled, the base of each cap lip was fit over a race track which projected upwardly from the sled. A series of attachment holes through the sled were located around the race track for the elastomeric material to seep through during the onsert molding process, which then secured the lip to the sled upon curing. Thus, a region on the sled was dedicated to the race and attachment holes, increasing the overall width of the sled. In the past, sled width was not a problem because the inkjet cartridges were replaceable and they each carried a significant supply of ink. The overall width of these replaceable pens often ranged from 2 to 3.5 centimeters. Thus, the cartridges themselves, when installed in a carriage, were far wider than the width required to place caps side-by-side on a sled.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote stationary location within the printer. Since these permanent or semi-permanent printheads carry only a small ink supply, they may be physically more narrow than their predecessors, the replaceable cartridges.
Narrower printheads lead to a narrower printing mechanism, which has a smaller "footprint," so less desktop space is needed to house the printing mechanism during use. Narrower printheads are usually smaller and lighter, so smaller carriages, bearings, and drive motors may be used, leading to a more economical printing unit for consumers. Thus, there are a variety of advantages associated with these off-axis printing systems.
Indeed, in the extreme case, each of the nozzle sets (for black, cyan, magenta and yellow inks, for instance) may eventually be manufactured on a single piece of silicon substrate, with printhead sealing accomplished by a single cap. Such a simple capping system clearly would not suffer the problems encountered when trying to seal several small discrete off-axis pens, each having their own silicon substrate printhead and the associated misalignment problems discussed above. Thus, the challenge becomes one of how to adequately cap several closely spaced discrete semi-permanent printheads. Proper capping requires providing an adequate hermetic seal without applying excessive force which may damage the delicate printheads or unseat the pens from their locating datums in the carriage. Moreover, it would be desirable to provide such a capping system which is more economical to manufacture than earlier capping systems. Such economies may be realized by requiring fewer parts for the capping system. It would also be desirable for such an improved capping system to be readily adaptable to the earlier mechanisms for moving caps in contact with the printheads.